Flashing Liquids Source Models

Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole Isothermal Flashing through hole Liquid pool boiling

Flashing Liquids We have considered source models in terms of liquids leaking through a hole or pipe and vapors leaking through a hole or pipe. For liquids stored under pressure above their normal boiling points, we need to consider flashing.

Adiabatic Flashing Liquids stored under pressure above normal boiling point. Large release of pressure (i.e. ruptured vessel). Energy to vaporize comes from liquid

Adiabatic Flashing Excess energy in superheated liquid Separate variables

Adiabatic Flashing Cp,l & hv are functions of T. If you assume they are constant at an average value

Adiabatic Flashing Determining the fraction of liquid vaporized Substitute back in average between Tb and T1

Adiabatic Flashing Design equation for fraction vaporized

Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole Isothermal Flashing through hole Liquid pool boiling

Adiabatic Flashing through holes Liquids stored above saturation pressure

Adiabatic Flashing through holes If L < 10 cm, assume incompressible liquid is flowing. If L>10 cm, assume choked flow with P2=Psat. Then design equation becomes: Where Psat is at ambient conditions

Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole Isothermal Flashing through hole Liquid pool boiling

Isothermal Flashing through a hole For liquids stored at saturation pressure, P1=Psat. Assume choked two-phase mass flow v is specific volume (1/density)

Isothermal Flashing through a hole The two-phase specific volume is vfg is difference in specific volume between liquid (fluid) and vapor (gas) vf is the liquid (fluid) specific volume fv is the mass fraction of vapor

Isothermal Flashing through a hole Differentiate with respect to pressure From before we determined

Isothermal Flashing through a hole All vapor formed is from liquid Substitute in

Isothermal Flashing through a hole Now substituted dfv into dv/dP relationship Clausius-Clapyron equation give dT/dP

Isothermal Flashing through a hole Substitute in the inverse of the Clausius-Clapyron relationship Substitute into final relationship

Isothermal flashing through holes Reduce to get design equation for vapor mass flow rate flashing through a hole When flashing at or near Psat small droplets of liquid are entrained with the vapor. Typically design assumption is that liquid mass is the same as the mass of the vapor formed from flashing

Flashing Liquids Adiabatic Flashing Adiabatic Flashing through hole Isothermal Flashing through hole Liquid pool boiling

Liquid Pool Boiling or Evaporating Use same relationship derived previously for evaporation Where K the mass transfer coefficient is estimated from

ChE 258 Chemical Process Safety In Class Problem Calculate the mass flux (kg/m2s) of sulfur dioxide that is leaking from a storage tank that holds liquid sulfur dioxide at its vapor pressure at 25°C Vapor pressure=0.39x106Pa Heat of vaporization=3.56x105J/kg vfg=0.09m3/kg Heat capacity=1.36x103J/kgK

Solution Use relationship derived in class Flux is

Solution cont. Substitute in values

Solution cont Finish reducing the units

Solution continued If we assume that entrained liquid droplets are being carried out with the flashing liquid then Total flux

ChE 258 Chemical Process Safety In Class Problem Calculate the mass flux (kg/m2s) of sulfur dioxide that is leaking from a storage tank that holds liquid sulfur dioxide at 300 psia and at 25°C. The wall thickness is 15 cm. Vapor pressure at 25 °C =0.39x106Pa Heat of vaporization=3.56x105J/kg Heat capacity=1.36x103J/kgK Liquid density=1.455gm/cm3

Solution Use relationship derived in class Flux is

Solution continued Get common units

Solution cont. Substitute in values

Solution Continued C0 has value of 0.61 for sharp edges, 1.0 for worst case Approximately 10 times greater than when stored at saturation pressure